1. Technical Field
The present invention relates to a curable composition which may be cured with high sensitivity by the application of energy, such as exposure to laser light, an image forming material which includes the curable composition in a recording layer and which allows direct drawing thereon with high sensitivity by a laser beam, and a negative-working planographic printing plate precursor in which the image forming material is used.
2. Related Art
Conventionally, a PS plate having a lipophilic photosensitive resin layer provided on a hydrophilic support has been widely used as a planographic printing plate precursor, and a desired printing plate is obtained by a plate-making method which usually involves mask exposure (surface exposure) via a lithographic film and then removal of non-image regions by dissolving. In recent years, digitalization techniques which involve electronic processing, accumulation and output of image information with a computer have been spreading. A wide variety of new image output systems compatible with such digitalization techniques have come to be practically used. As a result, there has been demand for computer-to-plate (CTP) techniques of producing a printing plate directly by scanning a highly directional light, such as laser light, according to digitalized image information, without employing a lithographic film. It has been a critical technical issue to provide a planographic printing plate precursor suitable for these techniques.
As a negative-working planographic printing plate precursor capable of such scanning exposure to light, a planographic printing plate precursor has been proposed which includes, on a hydrophilic support, a photopolymerizable photosensitive layer including a photopolymerization initiator, an addition polymerizable ethylenically unsaturated compound, and a binder polymer having a specifically structured repeating unit which is soluble in an alkali developer, and, according to necessity, a oxygen blocking protective layer (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2004-318053). In addition, a negative-working planographic printing plate precursor has been proposed which includes a photopolymerizable photosensitive layer and an oxygen blocking protective layer including an inorganic laminar compound, provided on a hydrophilic support (see, for example, JP-A No. 11-38633).
In the negative-working planographic printing plate precursor having a photopolymerization curable photosensitive layer, a radical polymerization reaction is inhibited in the presence of oxygen, which is likely to cause problems such as reductions in sensitivity and changes in sensitivity due to changes in oxygen concentration. In general, in order to prevent the occurrence of such problems, an oxygen-blocking protective layer is provided on the surface of the photosensitive layer, or the surface of the photosensitive layer is covered with a covering sheet as described above. However, the formation of the protective layer or the covering sheet increases the number of processes, and moreover, the protective layer and the covering sheet need to be thoroughly removed upon producing a plate from the planographic printing plate precursor, causing problems such as an increase in the number of processes, or an increase in the amount of developer used.
Therefore, attempts have been made to develop a negative photosensitive composition which does not require a protective layer, such as increasing the sensitivity of a recording layer. For example, attempts have been made to improve storage stability by adding an amine compound to quench oxygen, and techniques inhibiting the obstruction of polymerization due to oxygen, and the like, have also been proposed (for example, Japanese Patent Application Laid-Open (JP-A) No. 2002-274004 and JP-A No. 2006-264304). However, even when an amine compound such as the above is used, it has not been possible in practice to obtain curing properties which are completely problem-free. Moreover, a low molecular weight amine compound in particular has posed problems with respect to handling properties, in that a low molecular weight amine compound is likely to volatilize while heating, or the low molecular weight amine compound emits an odor at the time of the formation of a cured coating film, and so on.
Therefore, a curable composition has been demanded which is cured with high sensitivity by exposure to light and allows problem-free handling in practice.